1. Field of the Invention
This invention is in the field of semiconductor materials.
2. Description of the Prior Art
Photovoltaic cells have been developed which generate electrical energy directly from sunlight. Typically, such cells have been fabricated from semiconductor materials containing a rectifying junction, such as a p-n junction or a Schottky barrier.
A number of problems have been encountered, however, in attempts to produce photovoltaic cells which would have wide acceptance for use in producing energy from sunlight. One of the problems has been the cost of producing such cells, which has heretofore been relatively high. In fact, the cost of photovoltaic cells has generally been considered to be severely limiting except in applications where cost is not a controlling factor, such as in space applications or generation of power in remote areas.
The high cost of producing photovoltaic cells is due in large measure to the requirement for near crystal perfection and to the elaborate procedures involved in semiconductor wafer preparation. In regard to crystal perfection, it has almost universally been widely believed that only single crystal semiconductor materials could produce reasonable cell efficiencies. Polycrystalline semiconductor materials, on the other hand, have generally been unacceptable because they invariably have certain imperfections. Among these imperfections are the intersections between crystals, which are known as grain boundaries. Often, polycrystalline semiconductors also contain line imperfections within the crystals, which are known as dislocations. Still other possible imperfections include clusters, inclusions, impurities, etc.
Both grain boundaries and dislocations are known to possess properties which are different from the bulk crystal properties, including their electrical properties. Often, their resistance is lower than that of the bulk crystal, which produces a shorting effect through the polycrystalline semiconductor material when such materials are used in photovoltaic cells. The result is a low open circuit voltage which produces a concomittant reduction in photovoltaic cell efficiency.
Diffusion of impurities, such as a contact material, can also occur more rapidly into and along grain boundaries than into the bulk of semiconductor materials. Thus, the grain boundaries have a tendency to deteriorate more quickly than the bulk material itself.
Grain boundaries and dislocations additionally produce an overall degradation in semiconductor diode performance.
Because of the problems encountered with polycrystalline materials, they have not found wide acceptance for use in photovoltaic cells. Furthermore, those few photovoltaic cells which have employed polycrystalline materials have heretofore had much lower cell efficiencies than corresponding cells based upon single crystal semiconductor materials. Thus, although polycrystalline materials can be produced relatively inexpensively compared to single crystal materials, they have not been employed to any great extent in photovoltaic cells and, even when employed, have produced unsatisfactory photovoltaic cell efficiencies.